1. Field of the Invention
The present invention relates to a lower layer structure for a polycrystalline semiconductor layer, and in particular to a lower layer structure for a polycrystalline silicon active layer of a thin film transistor (hereinafter referred to as a xe2x80x9cTFTxe2x80x9d) of an active matrix type display.
2. Description of the Related Art
Because the thickness, size, and weight of flat panel displays such as liquid crystal displays (hereinafter referred to as xe2x80x9cLCDsxe2x80x9d can be reduced and the flat panel displays have low power consumption, LCDs or the like are now widely used as displays for various devices such as, for example, portable information devices. LCDs having a thin film transistor or the like provided as a switching element in each pixel are called xe2x80x9cactive matrixxe2x80x9d displays. Because display content for each pixel can be reliably maintained in such panels, active matrix displays are used for high resolution, high quality display applications.
FIG. 1 shows an equivalent circuit for a pixel in an active matrix type LCD. Each pixel comprises a thin film transistor (TFT) connected to a gate line GL and a data line DL. When the TFT is switched on by a selection signal output through the gate line, data corresponding to the display content is supplied from the data line through the TFT to a liquid crystal capacitor Clc. Also, because the written display data must be reliably maintained for the duration of the time from after the TFT is selected and data is written until the TFT is selected again, a storage capacitor Csc is connected to the TFT in parallel to the liquid crystal capacitor Clc.
In such an active matrix type LCD, a top gate type TFT in which a polycrystalline silicon (polysilicon) layer is used as an active layer and a gate electrode is formed above the active layer is a known type of TFT provided for each pixel. Because a top gate type polysilicon TFT is self-aligned and a source region, a drain region, and a channel region can be easily formed in the polysilicon active layer using its gate, this type of TFT is very advantageous for reducing the size of the TFT and for integration of TFTs.
Moreover, it is known that a polycrystalline silicon layer can be formed by laser annealing through low temperature processes which can polycrystallize an amorphous silicon film after such an a morphous silicon film is formed. The laser annealing can be employed also for forming a high-quality polycrystalline silicon layer on a glass substrate having a low melting point which is inexpensive as a substrate and in which a large area can easily be secured. Accordingly, currently, the laser annealing process is used for manufacturing polysilicon TFTs for active matrix type LCDs.
When such a TFT is used in, for example, a projector panel, in some cases, a metal layer may be formed below the TFT as a light shielding member for preventing light from a light source from entering the active layer of TFT. Moreover, when the TFT is used in a high resolution panel or the like, a metal layer is in some cases formed as a black matrix below the TFT and around the pixel electrode.
Although the laser annealing process as described above allows formation of polycrystalline silicon having superior properties, there is a problem in that the quality of the polycrystalline silicon films resulting from the laser irradiation significantly varies depending on the materials below the silicon layer.
In a top gate type TFT, no structure is required below the channel region of the active layer. Thus, the annealing conditions for the channel formation region of the TFT having a metal layer formed below the channel formation region as described above may differ from those for the channel formation region of the TFT provided on the same substrate but having no metal layer below the channel formation region, because of the thermal conductivity or the like of the metal layer. Therefore, even when identical laser outputs of equal strength are irradiated onto amorphous silicon film, the actual annealing conditions may differ significantly because of the different materials in the layers below the TFT active layer.
FIG. 2A is a diagram showing the relationship between the energy output of a laser and the grain size of the polycrystalline silicon obtained by the laser annealing process. As shown in FIG. 2A, although until a certain point the grain size increases as the supplied energy increases, after the supplied energy exceeds the energy value at which the maximum grain size can be obtained, the grain size rapidly decreases as the supplied energy further increases.
When a metal layer is present below the amorphous silicon film, the heat generated by the laser diffuses very rapidly due to the presence of the metal layer having a relatively high thermal conductivity. In contrast, if the lower layer is a glass substrate, for example, the heat tends to escape more slowly, and, thus, the amorphous silicon film can be heated for a sufficient amount of time. The relationship between the energy supplied by the laser and the grain size which can be obtained respectively for an amorphous silicon film over a glass substrate and for an amorphous silicon film over a metal layer are shown in FIG. 2B. As is clear from FIG. 2B, when the thermal conductivity significantly varies below the active layer and the laser annealing process is to be applied simultaneously, if the laser energy is set so that a large grain size can be obtained at the region where a metal layer is present as a lower layer, for example, the laser at the region where no metal layer is present as a lower layer would be overly irradiated, and, thus, the grain size would be very small.
On the other hand, if the conditions are set to obtain a proper grain size at the region where no metal layer is formed below, the grain size in the polycrystalline silicon film over the region where the metal layer is formed would not be sufficient. Therefore, when thermal conductivities of underlying layers differ significantly, it is very difficult to set the conditions for forming a polycrystalline silicon film having a large grain size in all regions.
Accordingly, one object of the present invention is to provide a structure for forming a polycrystalline semiconductor film which is used as an active layer of a top gate type TFT or the like, wherein the properties of the film are appropriate in all regions.
In order to achieve at least the object described above, according to the present invention, there is provided a semiconductor device comprising a metal layer formed over a portion of a transparent substrate; a polycrystalline semiconductor film formed above and at least partially overlapping the metal layer and polycrystallized by laser annealing; and a buffer layer provided between the metal layer and the polycrystalline semiconductor layer.
According to another aspect of the present invention, there is further provided a semiconductor device comprising a metal layer formed over a portion of a transparent substrate; a first polycrystalline semiconductor film formed above the metal layer to at least partially overlap the metal layer and a second polycrystalline semiconductor film formed above the region where the metal layer is not formed, the first and second polycrystalline semiconductor films polycrystallized by laser annealing; and a buffer layer provided below the first and second polycrystalline semiconductor film layers and above the metal layer.
According to the present invention, a buffer layer is provided below the semiconductor film which is polycrystallized through laser annealing. In the present invention, it is preferable that the buffer layer has a function to alleviate thermal leakage caused by thermal conduction in the metal layer, through, for example, sufficient thickness and thermal capacity. Thus, even when the material of the layer further below the buffer layer is, for example, a metal layer or a substrate such as glass and there is a significant difference in the thermal leakage for laser annealing, the difference in thermal leakage (escape) can be alleviated by the buffer layer so that the semiconductor films above the buffer layer can be crystallized to form a polycrystalline semiconductor film with proper characteristics.
According to another aspect of the present invention, it is preferable that, in the semiconductor device, the polycrystalline semiconductor film forms an active layer of a thin film transistor.
According to yet another aspect of the present invention, it is preferable that, in the semiconductor device, the buffer layer comprises a silicon oxide film formed at the side near the polycrystalline semiconductor film with a thickness of 200 nm or greater and a silicon nitride film formed at the side near the transparent substrate with a thickness of approximately 50 nm.
According to still another aspect of the present invention, it is preferable that, in the semiconductor device, the buffer layer comprises a silicon nitride film formed at the side near the transparent substrate with a thickness of 100 nm or greater and a silicon oxide film formed at the side near the contact surface with the polycrystalline semiconductor film with a thickness of 130 nm or greater.
Because the buffer layer as described above has a sufficiently large thermal capacity, and a large interlayer distance can be provided between the active layer and the lower layers, thermal leakage caused by the lower layers can be prevented irrespective of the material of the lower layers and heat necessary for annealing the semiconductor layer above can be maintained. Also, because the light transmittance properties of a silicon oxide film and a silicon nitride film are comparable to those of the glass substrate, any variation in the transmittance of the device substrate caused by the formation of the buffer layer will be small. By providing, below the polycrystalline semiconductor film such as, for example, a polycrystalline silicon film, a silicon oxide film which is a material similar to that of the polycrystalline semiconductor film, application of unnecessary stress to the polycrystalline semiconductor film and, consequently, unnecessary defects can be avoided. Moreover, by providing a fine silicon nitride film as the film at the side of the buffer layer near the substrate, it is possible to prevent, for example, in a case where a low melting point glass is used as the substrate, entry of impurities such as alkali ion from the glass substrate into the semiconductor film.
According to a still further aspect of the present invention, there is further provided an active matrix type display, comprising a pixel section and a driver section on the same substrate; wherein a plurality of pixels are placed in the pixel section, each pixel comprising a pixel thin film transistor and a display element; the driver section comprises a plurality of driver thin film transistors for outputting signals for driving each of the pixels in the pixel section; the pixel thin film transistor and the driver thin film transistors both are formed as top gate type transistors on the substrate using polycrystalline silicon as the active layer, the material for the polycrystalline silicon being identical; a buffer layer comprising a silicon oxide film and a silicon nitride film is formed below the polycrystalline silicon active layers of the pixel thin film transistor and the driver thin film transistors; and a metal layer is placed below the polycrystalline silicon active layer of the pixel thin film transistor, with the buffer layer in between.
According to another aspect of the present invention, it is preferable that in an active matrix type display, each of the pixels further comprises a storage capacitor with the first electrode electrically connected to the active layer of the pixel thin film transistor; and the second electrode of the storage capacitor is formed by the metal layer.
When a polycrystalline silicon is used for the active layer of a top gate type thin film transistor in an active matrix type display, in addition to the pixel thin film transistors, drivers for driving the pixel section can be formed on the same substrate, as described above. When this is done, a storage capacitor having sufficient size can be provided while efficiently saving space within a pixel by forming electrodes of a storage capacitor for maintaining display data for a predetermined duration of time under the active layer of the pixel thin film transistor. Also, a light shielding metal layer may in some cases be formed to prevent incident light.
On the other hand, because high speed operation is desired for the drivers, it is desirable that no conductive layer be formed in the drivers which constitutes a capacitance component with the active layer. Even in such a case, according to the present invention, a buffer layer is formed under the active layers of both transistors. Because the buffer layer alleviates the difference in thermal leakage caused by the thermal conduction in the material under the buffer layer, it is possible to form the polycrystalline silicon active layers of the pixel thin film transistors and the driver thin film transistors which are polycrystallized through the same laser annealing process at proper film quality.
According to the present invention, by forming a buffer layer having sufficient thickness and thermal capacity between a transparent substrate such as glass and a semiconductor film polycrystallized through laser annealing, the thermal leakage under the buffer layer can be alleviated, and polycrystalline semiconductor films of appropriate quality can be formed under using one set of annealing conditions, regardless of whether or not a metal layer such as an electrode and a black matrix is present in a lower layer.